Cytochrome oxidase, the terminal respiratory complex of the electron transfer chain, has recently been shown to contain subunit polypeptides which are encoded in mitochondrial DNA and synthesized on mitochondrial ribosomes. Mutants of Saccharomyces cerevisiae deficient in cytochrome oxidase will be isolated and studied in order to saturate the mitochondrial genome with respect to mit genes that are required for the biosynthesis of the enzyme. At present three distinct genetic loci designated as OXI 1, OXI 2 and OXI 3 have been established to code for gene products of cytochrome oxidase. Mutants carrying mutations in each of these loci will be examined with the aim of identifying the defective gene products. Similar studies will be done with other mutants of Saccharomyces cerevisiae that are specifically deficient in cytochrome b.